This application is based on applications No. 10-231921 and No. 11-122268 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal composition which exhibits a cholesteric phase at room temperature and a reflective type liquid crystal light modulating device provided with the liquid crystal composition.
2. Description of the Related Art
Recently, studies have been made to develop reflective liquid crystal displays by using chiral nematic liquid crystal which is produced by adding a chiral agent to nematic liquid crystal to cause the liquid crystal to exhibit a cholesteric phase at room temperature. Such a liquid crystal display makes a display by switching between a planar state (color) and a focal-conic state (transparent) in accordance with the level of the pulse voltage applied thereto. The planar state and the focal-conic state can be maintained after the application of the pulse voltage (which is referred to as bistability or non-volatility), whereby the display can be maintained even after voltage is no longer applied.
At present, however, in such a reflective liquid crystal display, the reflectance is low, and the contrast between the planar state and the focal-conic state is not sufficiently high, and liquid crystal displays of this type which are satisfactory in performance such as color purity (excitation purity), etc. are yet to be developed. To this type of liquid crystal displays, it is also important to have a wide temperature range in which practical display performance is possible.
It is an object of the present invention to provide a liquid crystal composition which, when it is used for a liquid crystal light modulating device such as a liquid crystal display, is good in color purity, reflectance, etc., has a wide temperature range which permits practical display performance and is driven by a low voltage.
Another object of the present invention is to provide a liquid crystal light modulating device which is good in color purity, reflectance, etc., has a wide temperature range which permits practical display performance and is driven by a low voltage.
In order to attain the objects, a liquid crystal composition according to the present invention is a chiral nematic liquid crystal composition which exhibits a cholesteric phase in a room temperature and is produced by adding a chiral agent at a ratio within a range from 10 wt % to 45 wt % to nematic liquid crystal which contains a nematic liquid crystal component with a polar group at not less than 25 wt % and a transition temperature to the isotropic phase in a range from 70xc2x0 to 150xc2x0. The anisotropy of refractive index of the liquid crystal composition is 0.10 to 0.22, and the anisotropy of dielectric constant is 5 to 30.
A liquid crystal light modulating device according to the present invention comprises the above-described liquid crystal composition and a space maintaining member between a pair of substrates at least one of which is light transmitting.
As the nematic liquid crystal component with a polar group, nematic liquid crystal with a polar bonding group in the molecular structure and nematic liquid crystal with a polar group as a substituent can be used. For example, a liquid crystal ester compound with an ester bonding group, a compound with a polar group in the molecule such as a liquid crystal pyrimidine compound of a pyrimidine structure, a liquid crystal tolane compound, a liquid crystal ester compound, a liquid crystal pyrimidine compound or the like which has a polar group such as a fluorine atom, a fluoroalkyl group, a cyano group as a substituent can be named.
As the other component of the nematic liquid crystal, it is preferred to use nematic liquid crystal with a low viscosity. For example, a liquid crystal tolan compound with alkyl groups at both ends, a liquid crystal phenylcyclohexane (PCH) compound with alkyl groups at both ends, etc. can be named as the nematic liquid crystal with a low viscosity. Further, a liquid crystal polycyclic (tricyclic or more) compound with no polar groups may be added so as to control the transition temperature to isotropic phase, or a liquid crystal tolan compound with no polar groups or a liquid crystal polycyclic compound may be added so as to control the anisotropy of refractive index.
If the anisotropy of refractive index is too low, the quantity of scattered light will be small. Consequently, coloring in the planar state will be weak, and the reflectance will not be sufficient. On the other hand, if the anisotropy of refractive index is too high, the quantity of scattered light will be too large, and the transparency or the black display in the focal-conic state will be bad. Consequently, the display performance will be poor. According to the present invention, the anisotropy of refractive index is regulated to a value within a: range from 0.1 to 0.22. Thereby, both the color portions and the transparent portions are good in reflectance, and satisfactory contrast can be obtained.
Also, the anisotropy of dielectric constant is regulated to be not more than 30. Thereby, when a liquid crystal light modulating device is fabricated by use of the liquid crystal composition, the peripheral materials such as a sealing material are prevented from diffusing into the liquid crystal, and the reliability of the device can be improved. If the anisotropy of dielectric constant is less than 5, a high driving voltage will be necessary. If the transition temperature to isotropic phase of the nematic liquid crystal is lower than 70xc2x0 C., the temperature range which permits practical display performance will be narrow. If the transition temperature to isotropic phase is higher than 150xc2x0 C., the liquid crystal will be crystallized easily by addition of a chiral agent.